vfs: add read write files and caching #711

This adds new flags to mount, cmount, serve *

    --cache-max-age duration         Max age of objects in the cache. (default 1h0m0s)
    --cache-mode string              Cache mode off|minimal|writes|full (default "off")
    --cache-poll-interval duration   Interval to poll the cache for stale objects. (default 1m0s)

vfs/dir_test.go

@@ -282,7 +282,7 @@ func TestDirCreate(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, int64(0), file.Size())
 
-	fd, err := file.Open(os.O_WRONLY)
+	fd, err := file.Open(os.O_WRONLY | os.O_CREATE)
 	require.NoError(t, err)
 
 	// FIXME Note that this fails with the current implementation